I stand corrected.
Simply put, the Mil-Dot is a range estimating reticle that was developed for military applications. The space between the dot centers subtends one milliradian (Mil). One Mil subtends 3.6" at 100 yards, or 36" at 1,000 yards.
This reticle was developed in the late 1970s to help U.S. Marine snipers estimate distances, and is now standard for all military branches. The space between dot centers subtends one milliradian (mil) hence the name mil-dot. Contrary to popular belief it does not stand for "military dot". One mil subtends 3.6 inches at 100 yards or 36 inches at 1,000 yards. To use this system effectively you must know the size of the target. For instance most people are an average of 6 feet tall or 2 yards. The formula used for determining range to the target is (size of target x 1000 divided by number of mils the target covers).